TABLE 1: WIDTH TOLERANCES
DIMENSION
NOMINAL DESIGN TOLERANCES
FOR THE FOLLOWING TYPES OF DIMENSIONS (SEE ILLUSTRATIONS BELOW)
NORMAL WIDTH
WIDTH
WIDTH EQUALS SUM OF SEVERAL
SPACES
(WIDER TOLERANCES REQUIRED)
(NORMAL TOLERANCES)
Dimension "W" (Fig. 1) is not affected
by changes in the size of other width
dimensions and is therefore easier to control.
Dimension "W" (Fig. 2) is equal to the sum of
the spaces "A", "B", "C" & "D". Changes in
the size of any of the individual spaces
affects the width dimension, thus making it
harder to control.
INCHES
From
±
To
±
0
1
.020
.030
1
2
.030
.045
2
4
.040
.060
4
6
.050
.075
6
10
.060
.090
10
15
.075
.100
15
20
.090
.125
20
24
.125
.150
NOTE: The nominal tolerances are design tolerances. Once a part has been developed, the running tolerances can usually be
tighter than the design tolerances. Running tolerances normally must be ± the actual measurements obtained for development
samples.
Dimension "W" is not directly dependent on spaces "A"
through "D" as the reference dimension "REF" can
float. It is therefore easier to control than the "W"
dimension in fig. 2 and NORMAL TOLERANCES CAN BE
USED.
Dimension "W" is equal to the sum of the spaces "A"
+ "B" +"C" +"D" and is directly affected by changes
to these dimensions. It is therefore more difficult to
control than the "W" dimension in fig. 1. and WIDER
TOLERANCES ARE REQUIRED. (Dimension "E" in fig. 1
also is equal to the sum of the spaces.)
1"
3"E
REF
A
B
1"
C
1"
0"
1"D
A
W
3"
B
1"
C
1"
0"
1"D
W
3"
FIG. 1: NORMAL WIDTH
FIG. 2: WIDTH =THE SUM OF SEVERAL SPACES
2
TABLE 2: LEG LENGTH & HEIGHT DESIGN TOLERANCES
DIMENSION
NOMINAL DESIGN TOLERANCES
FOR THE FOLLOWING TYPES OF DIMENSIONS (SEE ILLUSTRATIONS BELOW)
LEG LENGTH OR
SINGLE SEGMENTS
SUM OF SEVERAL SEGMENTS
(NORMAL TOLERANCES)
(WIDER TOLERANCES REQUIRED)
HEIGHT
Dimension "L" (Fig. 3) is for a single segment.
The size of this segment is not dependent or
affected by changes in the size of other
dimensions and is easier to control.
Dimension "C" (Fig. 4) is for two or more
segments ("L"). Changes in the size of any of the
individual segments affects this dimension. This
type of dimension is harder to control.
INCHES
From
To
±
±
.125
.010
----––
.125
.250
.015
.020
.250
.500
.020
.030
1.0
.025
.040
1.5
.030
.045
1.5
2.0
.040
.060
2.0
3.0
.045
.070
3.0
4.0
.050
.075
4.0
6.0
.060
-------
6.0
8.0
.075
-------
8.0
10.0
.090
-------
.500
1.0
NOTE: The nominal tolerances are design tolerances. Once a part has been developed, the running tolerances can usually
be tighter than the design tolerances. Running tolerances normally must be ± the actual measurements obtained for
development samples.
Dimension "L" is for a single segment. It is not
dependent or affected by changes in the size of other
dimensions and easier to control than those in fig. 4.
NORMAL TOLERANCES CAN BE USED.
Dimension "C" is the sum of the segments "L" and is
affected by changes in these dimensions. It is
therefore more difficult to control "C" than the "L"
dimensions in fig. 3. and WIDER TOLERANCES ARE
REQUIRED.
L
1"L
0" L
0"L
C
1"
0"
C
1"
1"L
L
1"
0"L
FIG. 3: LENGTH OR HEIGHT IS A SINGLE SEGMENT
3
FIG. 4: SUM OF SEVERAL SEGMENT LENGTHS
TABLE 3: SPACE & GAP TOLERANCES
DIMENSION
NOMINAL DESIGN TOLERANCES
FOR THE FOLLOWING TYPES OF DIMENSIONS (SEE ILLUSTRATIONS BELOW)
SPACES & GAPS
GAP OR SINGLE SPACE
(NORMAL TOLERANCES)
MULTIPLE SPACES
(WIDER TOLERANCES
REQUIRED)
ADJUSTABLE GAP
(TIGHTER TOLERANCES
POSSIBLE)
Dimensions "S" or "G" (Fig. 5)
are not affected by changes in
the size of other dimensions and
are therefore easier to control.
Dimensions "A", "B", "C" & "D"
(Fig. 6) are multiple spaces that
control width "W". The greater
the number of spaces, the wider
the tolerances should be for the
individual spaces.
Dimension "G" (Fig. 7) can be
adjusted by moving the top leg
up or down instead of having to
change dimension "S". If this is
allowable, tighter tolerances are
possible.
±
±
INCHES
From
To
±
.060
.080
15 %
--------
--------
.080
.125
10%
--------
--------
.125
.250
10%
--------
.015
.250
.500
.025
.030
.015
.025
.035
.020
.500
1.0
1.0
2.0
.030
.045
.020
2.0
3.0
.040
.050
--------
3.0
4.0
.045
.060
--------
NOTE: The nominal tolerances are design tolerances. Once a part has been developed, the running tolerances can usually
be tighter than the design tolerances. Running tolerances normally must be ± the actual measurements obtained for
development samples.
Dimensions "A" + "B" +"C" +"D" are multiple spaces
that control the width "W". The greater the number
of spaces, the more difficult it is to control the "W"
dimension and to hold all the individual spaces within
spec. WIDER TOLERANCES ARE REQUIRED for the
spaces as well as the width.
S
0"
S
0"
S
SINGLE SPACES
A
G
0"
G
B
1"
C
1"
0"
1"D
W
3"
GAPS
FIG. 5: SINGLE SPACES & GAPS
FIG.6 7:
MULTIPLESPACES
SPACES
FIG
: MULTIPLE
4
EXAMPLE OF AN ADJUSTABLE GAP
0"S
0"
G
0"G
0"G
If the part is designed with the space "S" slightly greater
than the gap "G", the top leg can be moved up or down
slightly to adjust the gap dimension while running the part.
MINIMUM GAP SIZE
Do not specify gaps less than .060" as a jig must be placed
inside of gap to support the legs. The wider the leg, the
larger the gap should be as thicker jigs must be used for
deeper gaps.
1/8 APPROX
0"
GAPS FOR THIN INSERTS
If paper or thin flexible inserts are used, set the gap at
.060" and add a small thin bead as shown.
.030" APPROX.
0"
0"G
CONTROL OF OVERLAP
During development, if this leg is running short and the width of the
slot is running toward the max., standard SPC practice is to lengthen
this leg to insure sufficient overlap of the insert. If a minimum
overlap is required, this should be specified.
INFORMATION ABOUT INSERT & REQUIRED FIT
Specify the width, thickness and material of the insert and indicate how
it is to be inserted into the slot such as:
• Insert slides in from the end,
• Insert flexes and is inserted from the front,
• Insert must slide up into the top gap and then drop into the bottom
gap.
The bottom leg can normally be shorter.
0"
0"
GAP
PROBLEMS WITH DEPTH & FLATNESS
0"
DEPTH
LEG HEIGHT
TOP GAP
0"
0"
BOTTOM GAP
This design makes it difficult to control the depth as
the several leg heights won't be the same. This also
makes it difficult to get flat bottom and top surfaces.
STACKED GAPS CAUSE PROBLEMS
This is an example of "Stacked Gaps". It is difficult to adjust
the gap size, as whatever is done to the top gap usually
affects the bottom gap. Larger tolerances may be required,
especially if there are a series of gaps as shown. Changes in
the adjacent gaps makes it difficult to control the gap
dimension.
FIG. 7: GAP DESIGN
5
TABLE 4: ANGLE DESIGN TOLERANCES
DIMENSION
LENGTH OF
SHORT LEG
INCHES
NOMINAL DESIGN TOLERANCES
FOR THE FOLLOWING TYPES OF DIMENSIONS (SEE ILLUSTRATIONS BELOW)
NORMAL TOLERANCES
TIGHT TOLERANCES
See FIG 8:ANGLE TOLERANCES below and FIG. Tight tolerances should only be used for simple
9: PROBLEMS IN MEASURING ANGLES on the shapes such as angles and channels. See FIG
8:ANGLE TOLERANCES below and FIG. 9:
next page.
PROBLEMS IN MEASURING ANGLES on the
next page.
±
±
.25
5°
4°
.50
4°
3°
.75
3°
2°
1.0
2°
1.5°
1.5
2°
1.5°
2.0
1.5°
1°
3.0 & Up
1°
.75°
4"
3"
2"
1"1"
0"
X
LINEAR DISPLACEMENT
OF SHORT LEG DUE TO
CHANGE IN ANGLE "A"
45°
A SPECIFIED ANGLE
Angles for plastic extrusions are normally
controlled by forming plates and jigs as the part is
being run. It is usually difficult to move a leg only
a few thousandths in order to correct for the linear
displacement "X" shown in the illustration. The
legs also move laterally and vertically a small
amount due to the extruder surging or due to air
drafts as the part is being formed. When legs are
short, a small linear displacement changes the
angle significantly. This is why the angular
tolerance (in degrees) that can be held increases as
the length of the short leg decreases. For a 1°
change in an angle, the linear displacement for a 1"
long leg is only .017". This jumps to .070" for a
4" long leg.
FIG. 8: ANGLE TOLERANCES
6
This is a drawing of a part. All the legs are
perfectly straight and it is easy to measure
the angles.
The legs on plastic extrusions are not
perfectly straight but are usually curved
slightly as shown below. Note the difference
in the measurement at "A" compared to that
at "B". For all practical purposes, the angle
between the legs is at 120° and the
measurement at "A" is correct.
Sometimes it is better to use linear
measurements as shown below to control
angles.
A
B
120°
120°
60°
113°
60°
1"
1"
2"
2"
FIG. 9: PROBLEMS IN MEASURING ANGLES
2"
2"
1"
1"1"
Angles for plastic extrusions are normally
controlled by forming plates and jigs as the part is
TABLE 5: FABRICATION
TOLERANCES
being run. It is usually difficult to move a leg only
a few thousandths in order to correct for the linear
displacement "X" shown in the illustration. The
+/- .008
HOLE SIZE
legs
also move laterally and vertically a small
amount due to the extruder surging or due to air
+/- .031
EDGE TO FIRST HOLE
drafts
as the part is being formed. When legs are
0"
X
short, a small linear displacement changes the
DISPLACEMENT
+/- .031
HOLE TOLINEAR
HOLE LOCATION
angle significantly. This is why the angular
OF SHORT LEG DUE TO
tolerance
(inLENGTH
degrees)TOLERANCE
that can be held increases as
VARIES W/
LAST HOLE
TO EDGE
CHANGE
IN ANGLE "A"
the length of the short leg decreases. For a 1°
45°
A SPECIFIED
ANGLE
+/- .031in an angle, the linear displacement for a 1"
NOTCH
WIDTH & HEIGHT
change
long leg is only .017". This jumps to .070" for a
4" long leg.
FIG. 8: ANGLE TOLERANCES
7
FIG. 10: DEGREES OF DIFFICULTY
EASY
DIFFICULT
EASY
DIFFICULT
BEST
EASY
CAUSES BOW
ALMOST
IMPOSSIBLE
DIFFICULT
IMPOSSIBLE
DETAILS
8
DIMENSIONING
W
DIMENSION NOT PARALLEL TO POINTS BEING MEASURED
(OK FOR OVERAL DRAWING DIMENSION BUT DIFFICULT TO MEASURE
OK
A
B
OUTSIDE TO INSIDE DIMENSIONS (A & B) DIFFICULT TO MEASURE
W
DIMENSIONS FORM INTERSECTION OF GEOMETRICAL POINTS
(DIMENSION OK AS REFERENCE DIMENSION BUT NOT MEASURABLE
DATUM LINE DIMENSIONING
(NORMALLY CAN MEASURE, BUT NOT WHAT WANT TO MEASURE)
9
FACTORS AFFECTING TOLERANCES
• TOLERANCES DEPENDENT UPON COMPLEXITY OF THE PART
IF MANY SEGMENTS MORE DIFFICULT TO HOLD TOLERANCES
IF SIMPLE PART CLOSER TOLERANCES CAN BE HELD
• CHANGING PROCESS CONDITIONS
AMBIENT TEMPERATURE
CHANGES IN MATERIAL FROM BATCH TO BATCH
• OPERATOR DEPENDENT -- MANY MANUAL ADJUSTMENTS
• DIE DESIGN
GUIDELINES FOR USING TOLERANCE TABLES
• TOLERANCES ARE NOMINAL
• BEST TO USE DIMENSIONS THAT CAN BE MEASURED DIRECTLY
• START WITH DIMENSIONS AFFECTING FIT WITH MATING PARTS
• TRY TO LIMIT NUMBER OF DIMENSIONS WITH TOLERANCES
• USE REFERENCE DIMENSIONS WHERE POSSIBLE
SPC DEFINITION OF REFERENCE DIMENSION
REFERENCE DIMENSIONS USED IN DIE DESIGN
REFERENCE DIMENSION CHECKED DURING DEVELOPMENT
THICKNESS
• NORMALLY SPECIFY NOMINAL THICKNESS
EXPECT THICKNESS VARIATIONS
• NOMINALLY -- TO .090" ± 10%, OVER .090" ± 8%
• WHERE REQUIRED -- SPECIFY MIN OR MAX FOR PARTICULAR AREA
RADII
• NORMALLY NOT TRUE RADII --DIFFICULT TO MEASURE DIRECTLY
• USE RADIUS GAUGES WHEN NEED TO MEASURE
• WHEN TOLERANCES REQUIRED SEE TABLE 2 (SINGLE SEGMENT)
LENGTH
• NORMALLY ± 1/16"
• FACTORS AFFECTING LENGTH CONTROL
LENGTH, LINE SPEED
CHANGE IN LENGTH DUE TO TEMPERATURE --SPC TO DESTINATION
10